For cutting tools, cemented carbide (WC—Co alloy, or the same having carbon nitride such as Ti (titanium), Ta (tantalum), or Nb (niobium) added to) has been conventionally used. However, in accordance with the trend of high speed cutting in the recent years, cutting tools covered with at least one layer of a coat by using cemented carbide, cermet, sintered cubic boron nitride, or alumina type or silicon nitride type ceramic as the base material, and applying chemical vapor deposition (CVD) or physical vapor deposition (PVD) on the surface has become pervasive. The coat layer has a thickness of 3-20 μm, and is formed of a compound including a first element of at least one type selected from the IVa group element, Va group element, VIa group element from the periodic table of elements, Al (aluminum), Si or B, and a second element of at least one type selected from B, C, N or O (when the first element is B alone, the second element excludes B).
The demand for saving energy and reducing the cost in modern cutting machinery is high. Accordingly, the demand for further increase in speed and feeding, as well as high efficiency in machining is elevating in terms of the cutting machining conditions. Further, the trend is towards low cost, high strength, and lightness in the machine components. The workpiece material employed in components has become difficult to be cut, rendering difficulty in the machining of the workpiece material. In view of the foregoing, various approaches have been taken to increase the chipping resistance and lifetime of cutting tools.
As an approach directed to increasing the lifetime of a cutting tool, Japanese Patent Laying-Open No. 2001-062603 (Japanese Patent No. 3661503, Patent Literature 1), for example, discloses a coat film of an oblong crystal texture, having residual stress differing between the upper section and lower section of the coat film. Specifically, residual compressive stress is applied to the upper layer of the coat film while residual tensile stress is applied to the lower layer of the coat film, whereby the chipping resistance of the cutting tool is improved.
Japanese Patent Laying-Open No. 2001-096404 (Patent Literature 2) discloses the approach of improving chipping resistance by covering with a hard coat layer including the stack of a Ti compound layer, a titanium carbonitride layer, a titanium carbide layer, and an aluminum oxide layer.
Japanese Patent Laying-Open No. 2009-078309 (Patent Literature 3) teaches improving the chipping resistance by introducing compressive stress towards the outer layer side of a hard coat layer covering the surface of a surface-coated cutting tool by means of blasting.
Further, WO2006/064724 (Patent Literature 4) discloses a coat film including a first coat formed of α-Al2O3 to which compressive stress is applied at the surface side and a second coat formed of TiCN to which tensile stress is applied at the inner side. This coat film has the toughness and wear resistance improved by setting the tensile stress of the first coat and the compressive stress of the second coat to satisfy a predetermined relationship.